This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Hydrocarbons are generated in the subsurface from source rocks rich in organic matter. Following initial deposition, source rocks are buried and subjected to increasing temperature and pressure with increasing burial. Hydrocarbons are generated when the source rocks reach temperatures sufficient for the thermal conversion of organic material to kerogen and then to free liquid and/or gaseous hydrocarbon phases, which is a process called source rock maturation. Upon generation, the hydrocarbons may subsequently be expulsed from the source rock and migrate in the subsurface to reservoir rocks (such as sandstones or limestones) that have sufficient porosity, structure and an adequate seal that make them capable of trapping the hydrocarbon phase(s), allowing hydrocarbons to accumulate. Alternatively, hydrocarbons may migrate to a surface location (e.g., sometimes referred to as a seep). Any hydrocarbons present in the subsurface may be preserved or they may be subjected to different forms of alteration. For example, biodegradation is the process of degradation or consumption of hydrocarbons by micro-organisms. Similarly, hydrocarbons may be thermally altered by exposure to temperatures above their thermal stability. Alternatively, hydrocarbons may be oxidized or consumed in processes, such as thermochemical sulfate reduction.
Conventional practice uses molecular geochemistry analysis of hydrocarbon compounds in oil samples or gas compositional and stable isotope analysis of hydrocarbon compounds in gaseous samples. These techniques are capable of estimating the maturity of the source rock when hydrocarbons were generated, the source facies from which the hydrocarbons were generated (e.g., marine or terrestrial source rocks), and can sometimes be used to differentiate between different potential origins of hydrocarbons (e.g., biogenic as compared to thermogenic) and provide information on alteration. For example, a series of hydrocarbon composition and stable isotope models were developed to estimate thermal maturity and identify alteration in hydrocarbon gases. See e.g., Stahl, W. J., (1977), “Carbon and Nitrogen Isotopes in Hydrocarbon Research and Exploration”, Chemical Geology, Vol. 20, pp. 121-149; Berner, U., et al. (1988), “Maturity Related Mixing Model for Methane, Ethane and Propane, Based on Carbon Isotopes”, Advances in Organic Geochemistry, Vol. 13, pp. 67-72; Chung, H. M., et al., (1979), “Use of Stable Carbon Isotope Compositions of Pyrolytically Derived Methane as Maturity Indices for Carbonaceous Materials”, Geochimica et Cosmochimica Acta, Vol. 43, pp. 1979-1988; James, A. T., (1990), “Correlation of Reservoired Gases Using the Carbon Isotopic Compositions of Wet Gas Components”, AAPG Bulletin, Vol. 74, No. 9, pp. 1441-1458; Whiticar, M. J., (1996), “Stable isotope geochemistry of coals, humic kerogens and related natural gases”, International Journal of Coal Geology, Vol. 32, pp. 191-215.
Yet, conventional techniques have deficiencies. For example, these conventional techniques do not provide accurate quantitative estimates of hydrocarbon generation temperature. Furthermore, the conventional techniques do not provide an evaluation of the time at which hydrocarbons were first expulsed or migrated in the subsurface, the temperatures at which hydrocarbons are stored in the reservoir or when hydrocarbons underwent mixing (e.g., during migration or in reservoir). Similarly, the conventional techniques do not provide quantitative constraints on how much alteration hydrocarbons have experienced (e.g., from either biodegradation or thermal cracking), and do not provide the temperature and/or time at which alteration occurred, or for how long these processes were taking place.